1. Field of the Invention
The present invention relates to a thin film multilayer capacitor and a mounting method therefor. More particularly, the present invention relates to a thin film multilayer capacitor which is small and has a relatively large capacitance, and a mounting method therefor.
2. Description of the Related Art
In recent years, in connection with the movement toward a circuit having a higher density in the field of electronic parts, demand for further miniaturization and higher performance of a capacitor or the like has been increased. As a small capacitor, a multilayer ceramic capacitor or the like is known. A dielectric ceramic green sheet cut to a specified size is prepared for fabricating such a multilayer ceramic capacitor. This ceramic green sheet is subjected to printing with an electrode paste, is dried, is laminated, is compression-bonded and then is cut into a specified size followed by baking in order to form a chip. The chip is coated with an external electrode paste and then baked to produce a multilayer ceramic capacitor.
However, if a multilayer ceramic capacitor is fabricated in such a method, it is impossible to make dielectric layers thinner than the particle size of a raw ceramic material powder. Besides, owing to the problems of short circuits and disconnection at electrodes caused by the defects of the dielectric layers, it is difficult, at the present level of technology, to produce a capacitor that has dielectric layers with a thickness of 3 xcexcm or less. Thus, there has been a limit in realizing a multilayer ceramic capacitor having a smaller size and a larger capacitance.
In order to solve such problems, a multilayer ceramic capacitor is proposed in Japanese Unexamined Patent Application Publication 56-144523, for example, in which a dielectric body portion is produced on a substrate by a sputtering method. A method for producing a thin film and electrode of Al2O3, SiO2, TiO2 or BaTiO3 with a sputtering method is disclosed.
However, since materials such as Al2O3, SiO2 and TiO2 have small dielectric constants, it is necessary to make the film thickness very small if the capacitance of a capacitor is to be increased, entailing problems related to the reliability of electronic devices such as leak current and dielectric breakdown voltage. Accordingly, use of a material with a high dielectric constant such as SrTiO3(Ba,Sr)TiO3, PbTiO3, Pb(Zr,Ti)O3 and Pb(Mg,Nb)O3 as well as BaTiO3 can be considered. However, when such a material with a high dielectric constant is used in order to obtain a high dielectric constant in the state of a thin film, it is necessary to employ a deposition method such as a MOCVD method or the like for improving the crystallinity of the thin film when depositing the film at a high temperature, and since most of these materials having high dielectric constants are obtained by utilizing a solid sublimation technology, it is difficult to obtain a material having a high dielectric constant with good reproducibility at the time of lamination.
Furthermore, these thin films have a low mechanical strength. Thus, when a thin film multilayer capacitor that is a conventional multilayer ceramic capacitor, in which ceramic green sheets are laminated, is used as a chip part, there is a problem that it tends to be damaged. This is because it is necessary to move the capacitor while holding it by its thin film side when the capacitor is to be bonded to a wiring substrate at the substrate side. To solve such a problem, it is conceivable to form solder bumps on the surface side of a thin film which is opposite to the substrate side, and to move the thin film multilayer capacitor over to a wiring substrate while holding it by the substrate side, so as to mount it on the wiring substrate with the solder bumps.
However, in order to make progress in the movement toward miniaturization and height reduction of a thin film multilayer capacitor, it is necessary to make the substrate and the solder bumps as thin as possible, and in accordance with this, there is a possibility of damaging the substrate itself as a result of an external stress when the thin film multilayer capacitor contacts the wiring substrate in the course of the mounting. Furthermore, from the viewpoint of height reduction, it is desirable to hold the thin film multilayer capacitor which is supported by solder bumps in a configuration approximately parallel with the wiring substrate.
Accordingly, it is one of the primary objects of the present invention to provide a thin film multilayer capacitor that is small and thin, can provide a large capacitance, and is hard to damage in the course of mounting it on a wiring substrate.
Also, it is another object of the present invention to provide a thin film multilayer capacitor mounting method for mounting such a thin film multilayer capacitor on a wiring substrate.
The present invention is a thin film multilayer capacitor comprising a substrate and a laminated body made of a plurality of dielectric layers and electrode layers formed on the substrate, wherein at least three solder bumps for external connection are formed on the surface of the laminated body which is opposite to the substrate side.
In such a thin film multilayer capacitor, the electrode layers comprise a first group of electrode layers and a second group of electrode layers which are electrically divided by the dielectric layers, wherein the electrode layers of the first group are laminated with the electrode layers of the second group in an alternate manner, having the dielectric layers intervening therebetween, the dielectric layers being formed partially over the electrode layers, so that a structure can be realized in which the plurality of electrode layers of the first group are electrically connected with each other at portions where the dielectric layers are not formed, and the plurality of electrode layers of the second group are electrically connected with each other at the other portions where the electrode layers are not formed.
Furthermore, a protective film having openings is formed on the surface of the laminated body so that solder bumps can be formed with solder applied to connect to the electrode layers at the openings.
Furthermore, the dielectric layers are made of an oxide thin film comprising at least Ba or Sr which is preferably deposited by an MOCVD method using a dipivaloylmethanate complex adduct with triethylenetetramine or tetraethylenepentamine as a raw material.
The present invention also includes a thin film multilayer capacitor mounting method for mounting any of the thin film multilayer capacitors described above on a wiring substrate, wherein solder bumps are connected to the wirings on the wiring substrate.
By forming at least three solder bumps on a laminated body made of dielectric layers and electrode layers formed on a substrate, the surface of the laminated body which is opposite to the substrate side can be attached onto a wiring substrate. Accordingly, a thin film multilayer capacitor can be moved to the wiring substrate while holding it by the substrate side. Furthermore, by connecting at least three solder bumps to wirings on the wiring substrate, it is possible to mount a thin film multilayer capacitor on the wiring substrate in a state parallel with it, by which height reduction can be realized in the mounting. Furthermore, since solder bumps make it possible to mount a thin film multilayer capacitor on a wiring substrate in a state parallel with it, the thin film multilayer capacitor can be prevented from contacting the wiring substrate, and, therefore, damage on the thin film multilayer capacitor by an external stress can be prevented.
Furthermore, the area in which the electrode layers of the first group and the electrode layers of the second group face each other, is made larger by laminating the electrode layers of the first group and the electrode layers of the second group with dielectric layers intervening therebetween, resulting in a capacitor with a large capacitance.
Furthermore, the laminated body can be protected by forming a protective film, and by forming openings on the protective film, solder can be applied onto the laminated body to form solder bumps. Hereupon, it is preferable to form the openings and the solder bumps in a circular form, and it is desirable to strictly control the amount of solder for use in the solder bumps.
Furthermore, the dielectric layers are made of a member comprising at least Ba or Sr, and when deposition of the member is performed by a MOCVD method, using, as a raw material, a dipivaloylmethanate (DPM) complex adduct with triethylenetetramine or tetraethylenepentamine, M(DPM)2(tetraene)2 or M(DPM)2(triene)2; M=Ba, Sr, it can be used at a temperature not less than the melting temperature of the member, thus making it possible to vaporize it for conveyance, using a conventional bubbling method. Accordingly, the reproducibility at the time of deposition of the dielectric body is improved, and lamination of thin films with a high dielectric constant is made possible.
Furthermore, by using solder bumps to mount such a thin film multilayer capacitor on a wiring substrate, it is possible to move the thin film multilayer capacitor while holding it by the substrate side. Therefore, damage to the thin film multilayer capacitor can be prevented.